On Wed, May 10, 2023 at 08:28:17AM -0700, Doug Anderson wrote:

Hi,

Hi Doug,

On Wed, Apr 19, 2023 at 3:57 PM Douglas Anderson [off-list ref] wrote:

quoted

This is an attempt to resurrect Sumit's old patch series [1] that
allowed us to use the arm64 pseudo-NMI to get backtraces of CPUs and
also to round up CPUs in kdb/kgdb. The last post from Sumit that I
could find was v7, so I called this series v8. I haven't copied all of
his old changelongs here, but you can find them from the link.

Since v7, I have:
* Addressed the small amount of feedback that was there for v7.
* Rebased.
* Added a new patch that prevents us from spamming the logs with idle
  tasks.
* Added an extra patch to gracefully fall back to regular IPIs if
  pseudo-NMIs aren't there.

Since there appear to be a few different patches series related to
being able to use NMIs to get stack traces of crashed systems, let me
try to organize them to the best of my understanding:

a) This series. On its own, a) will (among other things) enable stack
   traces of all running processes with the soft lockup detector if
   you've enabled the sysctl "kernel.softlockup_all_cpu_backtrace". On
   its own, a) doesn't give a hard lockup detector.

b) A different recently-posted series [2] that adds a hard lockup
   detector based on perf. On its own, b) gives a stack crawl of the
   locked up CPU but no stack crawls of other CPUs (even if they're
   locked too). Together with a) + b) we get everything (full lockup
   detect, full ability to get stack crawls).

c) The old Android "buddy" hard lockup detector [3] that I'm
   considering trying to upstream. If b) lands then I believe c) would
   be redundant (at least for arm64). c) on its own is really only
   useful on arm64 for platforms that can print CPU_DBGPCSR somehow
   (see [4]). a) + c) is roughly as good as a) + b).

It's been 3 weeks and I haven't heard a peep on this series. That
means nobody has any objections and it's all good to land, right?
Right? :-P

FWIW, there are still longstanding soundness issues in the arm64 pseudo-NMI
support (and fixing that requires an overhaul of our DAIF / IRQ flag
management, which I've been chipping away at for a number of releases), so I
hadn't looked at this in detail yet because the foundations are still somewhat
dodgy.

I appreciate that this has been around for a while, and it's on my queue to
look at.

Thanks,
Mark.